By Denis O’Connor

Wharfedale Naturalists Society

A MONTH ago I wrote of finding a bank vole and its nest beneath an old dustbin lid in the garden. I have not seen a vole since although that may change when I begin to refill the bird feeders when they may emerge to scavenge fallen seeds.

Bank and field voles are just two of the tiny mammals that occur in big numbers in our area but which are seldom seen because of their mainly nocturnal habits. They have many predators such as owls, foxes, stoats and weasels and are an integral part of the food chain, especially for the owls.

Vole numbers often fluctuate hugely over a three or four year cycle with these changes strongly influencing owl numbers and their breeding success. In past years with prolonged snow cover, as happened in Wharfedale in the winter of 2011 and the spring of 2013, owls find it difficult to find voles and even those that survive may be in too poor condition to breed that year.

The crucial importance of snow on vole and owl populations was brought home to me last year during a wildlife trip to Finland. We were told that two years previously, due to a very cold winter with snow that lasted well into the spring, vole numbers had crashed and owls had been unable to locate those remaining below the snow cover. Many owls had starved and it had taken another year for both voles and owls to return to strength.

Our owl guide explained that the numbers of their biggest owl, the Great Grey, pictured here on a nest tray provided to supplement natural nest sites, had recovered to the point where he had located 21 nests in an area of 10 by 30 kilometres, a population density only made possible by an explosion in the vole numbers.

The smallest of their owls, the Pygmy, also makes use of nest boxes that are often put up in twos and threes. A pygmy owl will use one for its nest and another to store dead voles as an insurance against times when prey is in short supply.

With global heating, periods of lying snow are becoming more unusual in Britain although that hazard has been replaced in some years by weeks of prolonged rain that can make rodent hunting difficult, again with disastrous productivity for nesting owls.

Whether affected by snow or rain, for the survival of owls voles are the key!